1. Field of the Invention
This invention relates to a method of installing an offshore platform, and more particularly, to a method of installing a superstructure of an offshore platform on the top portion of a jacket which is fixed to the seabed.
2. Description of the Related Art
The conventional method of installing an offshore platform will be explained with reference to FIGS. 1 to 8. An offshore platform is composed of a jacket which is secured to the seabed and a superstructure which is supported by a support frame on the jacket. The jacket 1, having its own buoyancy, is first transported to a position for fixing the jacket 1 by a floating crank 3 (FIG. 1) and after positioning, it is made to sink to the seabed by pouring water into the steel pipes 31 which constitute the frame of the jacket (FIG. 2). This jacket 1 is tall enough to protrude above the sea level at its top portion. Once the bottom portion of the jacket 1 reaches the seabed, it is fixed by a pile 2 driven from above the sea level, as is shown in FIG. 2. A platform support frame 3 for supporting the superstructure of the platform is next installed, as is shown in FIG. 3. This superstructure of the platform is divided into several modules 4, which are successively installed by a floating crane 33 on the support frame 3, as is shown in FIGS. 4 and 5. At the same time as the installation of the modules 4 hook-up work is started. "Hook-up work" means pipework, wiring work, work of fitting apparatus sent separately from the above-described units, and so on. Therefore an accommodation module 5 (with a helicopter deck) is installed as is shown in FIG. 6, and a drilling apparatus 6 and the like are installed, as is shown in FIG. 7. After the completion of the above-described mechanical assembly of the units, commissioning which consists of trials and test runs is conducted to confirm effective function of the facilities, as is shown in FIG. 8, and a proper run is thus made possible.
Since this series of processes is conducted at sea, the working circumstances are very severe, and entailing many risks and requiring much time and expense. In addition, many workers are forced to live at sea for a long time under bad conditions. In order to improve these problems, the tendency is to make modules larger in size to reduce the number of units of the superstructure of a platform.
The location for such platform are increasingly selected in deep water areas and since the price of a jacket for a deep water platform advances in geometric progression in proportion to the depth of the sea area, a demand has arisen to concentrate facilities, which could separately arranged in several platforms according to their functions in shallow water, into one platform. Consequently a platform is apt to be lofty and extremely large with various facilities, and hence there is an increasing tendency to employ larger and larger modules.
However, there is a limit to the feasible size of a floating crane from the physical and economical point of view, and, therefore, to the size of a module.